This invention relates to low adhesion backsize and release coatings of the type employed in connection with normally tacky and pressure-sensitive adhesive tape. The invention also relates to adhesive-coated sheet material provided with such coatings.
Normally tacky and pressure-sensitive adhesive (PSA) tape has been widely used for well over half a century. Products of this type, which typically feature a sheet backing coated on one side with an adhesive that adheres to a wide variety of surfaces upon the application of pressure alone, are often sold in roll form. To permit the roll to be unwound without the undesirable transfer of adhesive to the other side of the backing, it is customary to provide that surface with a low adhesion backsize (LAB), to which the adhesive bonds less firmly. Tape products may also be sold in pad form, e.g., as a stack of labels or the currently ubiquitous repositionable yellow note pads, each sheet of which has a band of adhesive adjacent one edge; the adhesive on adjacent sheets may be either along the same edge or along opposite edges, the latter providing a "Z-stack" construction for easier dispensing. Another related construction is a fanfolded web of the type shown in U.S. Pat. No. 4,768,810. An LAB is also customarily employed in each of the foregoing constructions to contact the adhesive and permit ready separation of adjacent layers.
Although the only requirements usually imposed on an LAB are that it provide an appropriate degree of release to the adhesive with which it comes in contact and that it not deleteriously affect the adhesive, there are circumstances in which it is important that the LAB also possess other characteristics. For example, when it is desired to print the back of tape having an LAB, the LAB must be receptive to the printing ink, and a number of patents describe products said to be suitable for such use. In other cases it may be desired to write on the LAB-coated surface with pens containing either solvent- or water-based inks, the resultant indicia remaining firmly bonded and resisting any tendency to smear. Because LAB coatings are typically hydrophobic and releasable, they are generally receptive to solvent-based inks, while pens containing water-based ink typically produce lines that are not only discontinuous but also tend to smear.
U.S. Pat. No. 4,624,893 describes release layers that comprise modified starch and an organic fluorine compound and are said to be receptive to both water- and oil-based inks. These compositions provide suitably low release values only for repositionable pressure-sensitive adhesives, which have an adhesion to glass of no more than about 15 N/dm.
Japanese laid-open applications 60-94485, 60-94486, and 60-155451 describe release coatings which are receptive to water-based ink, prepared from triblock polymers of silicone and poly(alkyleneoxides) functionalized with acrylic groups and cured onto the substrate with electron beam irradiation. This curing requirement complicates the coating process, requires specialized and expensive equipment, and embrittles such substrates as paper.
Japanese laid-open application 60-155452 discloses similar radiation-cured release coatings, also prepared from vinyl-functional triblock polymers of silicone and poly(alkyleneoxides), the preparation procedure including the use of odoriferous mercaptoalkyl silicones.
U.S. Pat. No. 4,728,571 (the disclosure of which is incorporated herein by reference), describes a release agent based on a copolymer made up of A, B, and C monomers, wherein
A is at least one free radically polymerizable vinyl monomer; PA1 B is at least one polar monomer copolymerizable with A, the amount of B being up to 30% of the total weight of all monomers, and PA1 C is a monomer having the general formula X--(Y).sub.n SiR.sub.(3--M) Z.sub.m wherein PA1 A is at least one free radically polymerizable vinyl monomer; PA1 B is at least one polar monomer copolymerizable with A, the amount of B present being sufficient to give the copolymer a hydrated T.sub.g between -15.degree. C. and +35.degree. C., the temperature difference between the hydrated T.sub.g and actual T.sub.g being at least 20.degree. C., and PA1 C is a monomer having the general formula X--(Y).sub.n SiR.sub.3--m Z.sub.m wherein
X is a vinyl group copolymerizable with the A and B monomers, PA2 Y is a divalent linking group where n is zero or 1; PA2 m is an integer of from 1 to 3; PA2 R is hydrogen, lower alkyl (e.g., methyl, ethyl, or propyl), aryl (e.g., phenyl or substituted phenyl), or alkoxy; and PA2 Z is a monovalent siloxane polymeric moiety having a number average molecular weight above about 1,000 and is essentially unreactive under copolymerization conditions. PA2 X is a vinyl group copolymerizable with the A and B monomers, PA2 Y is a divalent linking group where n is zero or 1, PA2 m is an integer of from 1 to 3; PA2 R is hydrogen, lower alkyl (e.g., methyl ethyl, or propyl), aryl (e.g., phenyl or substituted phenyl), or alkoxy; and PA2 Z is a monovalent siloxane polymeric moiety having a number average molecular weight above about 1,000 and being essentially unreactive under copolymerization conditions,
The A and B monomers are copolymerized to form a polymeric backbone to which the C monomer is grafted, the amount and composition of the C monomer (about 0.01-20% of total monomer weight, the C monomer having a molecular weight in of 1,000-50,000, preferably 5,000-25,000) being chosen such that the surface release value of the composition does not exceed about 50 N/dm. A surface coated with this copolymer is receptive to solvent-based inks but not to water-based inks.